For the past several years, extensive research has been devoted to the development of metal-second phase composites, such as aluminum reinforced with fibers, whiskers, or particles of carbon, boron, silicon carbide, silica, or alumina. Metal-second phase composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix of composites formed, utilizing powder metallurgy techniques. However, such composites typically suffer from poor ductility and fracture toughness, for reasons which are explained below.
Prior art techniques for the production of metal-second phase composites may be broadly categorized as powder metallurgical approaches, molten metal techniques, and internal oxidation processes. The powder metallurgical production of dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with an oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques, or conventional procedures such as ball milling, may be used to mix the powders. Standard powder metallurgy techniques are then used to form the final composite. Conventionally, however, the ceramic component is large, i.e., greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials, because their production is energy and capital intensive, and time consuming. Furthermore, production of very small particles inevitably leads to contamination at the particle surface, resulting in contamination at the particle-to-metal interface in the composite, which in turn compromises the mechanical properties thereof. Also, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, molten metal infiltration of a continuous skeleton of the second phase material has been used to produce composites. In some cases, elaborate particle coating techniques have been developed to protect ceramic particles from molten metal during molten metal infiltration and to improve bonding between the metal and ceramic. Techniques such as this have been developed to produce silicon carbide-aluminum composites, frequently referred to as SiC/Al or SiC aluminum. This approach is suitable for large particulate ceramics (for example, greater than 1 micron) and whiskers. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603 to Yamatsuta et al, hereby incorporated by reference. Because this technique necessitates molten metal handling and the use of high pressure equipment, molten metal infiltration has not been a practical process for making metal-second phase composites, especially for making composites incorporating submicron ceramic particles, where press size and pressure needs would be excessive and impractical.
The presence of oxygen in ball-milled powders used in prior art powder metallurgy techniques, or in molten metal infiltration, can result in a deleterious layer, coating, or contamination such as oxide at the interface of second phase and metal. The existence of such layers will inhibit interfacial binding between the second phase and the metal matrix, adversely effecting ductility of the composite. Such weakened interfacial contact may also result in reduced strength, loss of elongation, and facilitated crack propagation.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as copper containing internally oxidized aluminum. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. Although this technique is limited to relatively few systems, because the two metals must have a wide difference in chemical reactivity, it has offered a possible method for dispersion hardening. However, the highest possible concentration of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness and the like.
In U.S. Pat. No. 2,852,366 to Jenkins, hereby incorporated by reference, it is taught that up to 10 percent by weight of a metal complex can be incorporated into a base metal or alloy. The patent teaches blending, pressing, and sintering a mixture of a base metal, a compound of the base metal and a non-metallic complexing element, and an alloy of the base metal and the complexing metal. Thus, for example, the reference teaches mixing powders of nickel, a nickel-boron alloy, and a nickel-titanium alloy, pressing, and sintering the mixed powders to form a coherent body in which a stabilizing unprecipitated "complex" of titanium and boron is dispersed in a nickel matrix. Precipitation of a ceramic phase is specifically avoided.
In U.S. Pat. No. 3,194,656, hereby incorporated by reference, Vordahl teaches the formation of a ceramic phase, such as TiB.sub.2 crystallites, by melting a mixture of eutectic or near eutectic alloys. It is essential to the process of Vordahl that at least one starting ingredient has a melting point substantially lower than that of the matrix metal of the desired final alloy. There is no disclosure of the initiation of an exothermic second phase-forming reaction at or near the melting point of the matrix metal.
Bredzs et al, in U.S. Pat. Nos. 3,415,697, 3,547,673, 3,666,436, 3,672,849, 3,690,849, 3,690,875, and 3,705,791, hereby incorporated by reference, teach the preparation of cermet coatings, coated substrates, and alloy ingots, wherein an exothermic reaction mechanism forms an in-situ precipitate dispersed in a metal matrix. Bredzs et al rely on the use of alloys having a depressed melting temperature, preferably eutectic alloys, and thus do not initiate a second phase-forming exothermic reaction at or near the melting temperature of the matrix metal.
DeAngelis, in U.S. Pat. No. 4,514,268, hereby incorporated by reference, teaches reaction sintered cermets having very fine grain size. The method taught involves the dual effect of reaction between and sintering together of admixed particulate reactants that are shaped and heated at temperatures causing an exothermic reaction to occur and be substantially completed. The reaction products are sintered together to form ceramic-ceramic bonds by holding the reaction mass at the high temperatures attained. Thus, this reference relates to a product with sintered ceramic bonds suitable for use in contact with molten metal.
Backerud, in U.S. Pat. No. 3,785,807, hereby incorporated by reference, teaches the concept of preparing a master alloy for aluminum, containing titanium diboride. The patentee dissolves and reacts titanium and boron in molten aluminum at a high temperature, but requires that titanium aluminide be crystallized at a lower temperature around the titanium diboride formed. Thus, the patent teaches formation of a complex dispersoid.
In recent years, numerous ceramics have been formed using a process termed "self-propagating high-temperature synthesis" (SHS). It involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders, typically for the purpose of preparing ceramic powders or parts, absent a metal binder. The SHS process involves mixing and compacting powders of the constituent elements and igniting a portion of a green compact with a suitable heat source. The source can be electrical impulse, laser, thermite, spark, etc. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation at high temperatures, rather than bulk heating over long periods at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al, U.S. Pat. Nos. 3,726,643; 4,161,512; and 4,431,448 among others, hereby incorporated by reference.
In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from Groups IV, V, and VI of the Periodic System with a non-metal, such as carbon, boron, silicon, sulfur, or liquid nitrogen, and heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, absent a binder.
When the SHS process is used with an inert metal phase, it is generally performed with a relatively high volume fraction of ceramic and a relatively low volume fraction of metal (typically 10 percent and below, and almost invariably below 30 percent). The product is a dense, sintered material wherein the relatively ductile metal phase acts as a binder or consolidation aid which, due to applied pressure, fills voids, etc., thereby increasing density. The SHS process with inert metal phase occurs at higher temperatures than the in-situ precipitation process used in conjunction with the present invention, and is non-isothermal, yielding sintered ceramic particles having substantial variation in size.
U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal or alloy, such as an alloy of copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the reaction, resulting in an alloy comprising titanium diboride, titanium carbide, and up to about 30 percent binder metal. This reference, however, is limited to the use of Group I-B metals or alloys, such as copper and silver, as binders. Products made by this method have low density, and are subjected to subsequent compression and compaction to achieve a porosity below 1 percent.
U.S. Pat. No. 4,540,546 to Giessen et al, hereby incorporated by reference, teaches a method for rapid solidification processing of a multiphase alloy. In this process two starting alloys react in a mixing nozzle in which a "Melt Mix Reaction" takes place between chemically reactable components in the starting alloys, to form submicron particles of the resultant compound in the final alloy. The mixing and chemical reaction are performed at a temperature which is at or above the highest liquidus temperature of the starting alloys, but which is also substantially below the liquidus temperature of the final alloy, and as close to the solidus temperature of the final alloy as possible. While dispersion-strengthened alloys can be produced by this technique, there appear to be a number of inherent difficulties. First, processing is technically complex, requiring multiple furnaces. Second, efficient mixing is important if fine dispersions are to be consistently produced. Lastly, very high degrees of superheat will be required to completely dissolve the rapid solidification alloying elements in order to produce high loading of dispersoid, which necessarily accentuates particle growth, for example, in composites containing 10-20% dispersoid.
The present invention overcomes the disadvantages of the prior art noted above. More particularly, the present invention permits simplification of procedures and equipment compared to the prior art. For example, the present process obviates the need for multiple furnaces and mixing and control equipment because all of the constituents of the second phase are present in a single reaction vessel. The present invention also overcomes the need for forming multiple melts of components at very high melting temperatures. Further, high loading composites can be prepared without the necessity of achieving high levels of superheat in holding furnaces.
Applicants' invention also provides for a cleaner particle/metal interface compared with conventional metal-ceramic composites made by, for example, powder metallurgical techniques using separate metal and ceramic powders, because the reinforcing particles are formed in-situ.
The invention is further advantageous over conventional technology because it uses low cost starting materials. The invention also provides a mechanism for producing metal-second phase composites containing carbides and nitrides and a mechanism for producing mixed second phase precipitates, for example, mixed ceramic precipitates such as TiB.sub.2 /TiN, or mixed ceramic/intermetallic precipitates such as TiB.sub.2 /TiAl. Applicants' process may also produce dispersions of second phase materials not normally wet by metals, for example, TiO.sub.2 in aluminum. With these facts in mind, a detailed description of the invention follows.